Solder composition

ABSTRACT

A solder composition containing a lead-free SnZn alloy and a solder flux that contains at least an epoxy resin and an organic carboxylic acid. The organic carboxylic acid is dispersed in the solder composition as a solid at room temperature, and has a molecular weight of from 100 to 200 g/mol.

This application is a divisional application of co-pending U.S.application Ser. No. 10/482,056 filed Oct. 8, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lead-free solder composition whichdoes not require flux removal and which undergoes few changes with timein its printability and solderability.

2. Background Art

Most conventional flux for solder comprises rosin or rosin-modifiedresin with an activator such as organic acid or halide added thereto.Rosin is the main component of flux, and when it is diluted with solventto have a suitable viscosity, it may improve the printability of solderthat comprises it. As adhesive, in addition, rosin acts to temporarilyfix electronic parts to a printed circuit substrate to prevent them fromdropping or shifting. Rosin contains an active ingredient of abieticacid, and even the acid alone may be effective in some degree forsolderability. Rosin that may be used in cream solder flux includes, forexample, natural rosin, polymerized rosin, disproportionated rosin,hydrogenated rosin, maleic acid-modified rosin. However, rosin-basedflux that contains such rosin remains as a residue on printed circuitboards after using the flux to mount electronic parts, and, in manycases, the residue has caused substrate corrosion and migration. Inaddition, when the printed circuit board with the residue remainingthereon is encapsulated with resin (e.g., silicone gel, epoxy resin),the residue may cause curing failure in resin encapsulation and maytherefore have some negative influence on the resin's adhesiveness toand insulation from backboard. To remove the residue, the soldered boardis generally washed with a flon substitute or organic solvent. Atpresent, however, the washing agent is limited owing to environmentalproblems with flon and VOC.

An epoxy flux is a type of flux that does not cause substrate corrosionand migration and does not cause curing failure in resin encapsulation,even though its residue is not removed through washing. The epoxy fluxmainly comprises an epoxy resin, a carboxylic acid, an amine and athixotropic agent. When parts are mounted on a printed circuit board bythe use of cream solder that contains an epoxy flux comprising suchcomponents, the solder is so planned that the conductor surface may beactivated by the carboxylic acid in the stage of reflow-soldering. Atthe same time, the epoxy resin may react with the carboxylic acid tocure, and its curing may be finished when the reflowed solder hasadhered to the parts. After the solder has reflowed, the cured epoxyresin remains as a flux residue. Compared with the residue from ordinaryrosin-based flux, the cured epoxy resin residue does not interfere withthe adhesiveness of encapsulation resin to printed circuit boards eventhough it is not removed after soldering. Moreover, the parts-solderedboard may be directly encapsulated with resin and its insulation is good(See JP-A 2000-216300).

On the other hand, an SnPb alloy is generally employed as a solder alloyto be mixed with flux. The SnPb alloy serves well for soldering, and themelting point of its eutectic composition (63Sn37Pb) is 183° C. (low),and its soldering temperature is not higher than 250° C. Accordingly, itdoes not cause thermal damage to electronic parts not resistant to heat.However, solder not containing lead, that is, lead-free solder, iscurrently desired because of the current environmental problems causedby lead.

The alloy for lead-free solder includes Sn-based SnAg alloy and SnSballoy. Of SnAg alloys, the composition having the lowest melting pointis a eutectic composition of Sn3.5Ag with a melting point of 221° C. Thesoldering temperature of the solder alloy having this composition isfrom 260 to 280° C. and is considered to be high. When soldering iseffected at such a temperature, electronic parts not resistant to heatmay be thermally damaged, whereby their function may deteriorate or theymay break. Among SnSb alloys, the composition having the lowest meltingpoint is Sn5Sb, and its melting point is 235° C. on the solid phase linethereof and is 240° C. on the liquid phase line thereof, both consideredto be high. Therefore, its soldering temperature is from 280 to 300° C.and is much higher than that of the Sn3.5Ag alloy. For the same reason,electronic parts not resistant to heat may be thermally damaged withthis alloy.

Recently, lead-free SnZn alloy solder has become much noticed in theart, of which the melting point is lower than that of SnAg alloy andSnSb alloy. Of the SnZn alloy, for example, the eutectic composition isSn9Zn, and its melting point is 199° C., which is near the melting pointof SnPb eutectic solder. The soldering temperature of the SnZn alloy islower than that of SnAg alloy and SnSb alloy, and the SnZn alloy mayreduce thermal damage to electronic parts not resistant to heat.

However, when cream solder of lead-free SnZn alloy is prepared usingconventional rosin-based flux, its printability and solderability may bealmost the same as those of conventional cream solder immediately afterits preparation. However, for example, when stored at room temperaturefor a while, the viscosity of the cream solder increases and itsprintability worsens and, in addition, its solderability also worsens.Further with the lapse of time, the viscosity of the SnZn-based creamsolder increases more, and, as a result, the solder completely loses itsprintability. In that condition, even though a solvent is added to it tolower its viscosity and the thus-diluted solder is used for printing,the SnZn alloy powder therein can no longer melt and will be useless forsoldering.

On the other hand, even when an SnZn alloy-based cream soldercomposition is produced using an epoxy flux so that flux residue removalmay be omitted, it still has the same problems as those of rosinflux-containing solder with respect to viscosity, the printability andthe solderability thereof.

OBJECT AND SUMMARY OF THE INVENTION

An object of the present invention is to solve the above-mentionedproblems and to provide a solder composition that contains an epoxy fluxand a lead-free SnZn alloy. The solder composition undergoes few changeswith time in its viscosity, printability and solderability, notrequiring flux removal.

The printability and the solderability of the cream solder compositionthat contains a lead-free SnZn alloy and an epoxy flux change with timebecause the Zn component of the SnZn alloy is highly reactive. The Zncomponent may therefore react with the component of the epoxy flux,essentially with the organic carboxylic acid in the flux with the lapseof time. As a result, when the reactivity of Zn with the organiccarboxylic acid in the solder composition is suppressed, theabove-mentioned problem may be solved. The invention is based on thisfinding.

Specifically, the first embodiment of the invention is a soldercomposition containing a lead-free SnZn alloy and a solder flux thatcontains at least an epoxy resin and an organic carboxylic acid. In thisembodiment, the organic carboxylic acid is dispersed in the soldercomposition as a solid at room temperature (25° C.).

The second embodiment of the invention also is a solder compositioncontaining a lead-free SnZn alloy and a solder flux that contains atleast an epoxy resin and an organic carboxylic acid. In this embodiment,the organic carboxylic acid has a molecular weight of from 100 to 200g/mol. Preferably, the organic carboxylic acid of this embodiment isdispersed in the solder composition as a solid at room temperature (25°C.).

The third embodiment of the invention likewise is a solder compositioncontaining a lead-free SnZn alloy and a solder flux that contains atleast an epoxy resin and an organic carboxylic acid. In the embodiment,the organic carboxylic acid is in microcapsules that are encapsulatedwith a film selected from a group consisting of epoxy resin, polyimideresin, polycarbonate resin, polyamide resin, polyester resin, polyurearesin, polyolefin resin, and polysulfone resin.

The fourth embodiment of the invention also is a solder compositioncontaining a lead-free SnZn alloy and a solder flux that contains atleast an epoxy resin and an organic carboxylic acid. The SnZn alloy ofthis embodiment is in microcapsules that are encapsulated with a filmselected from a group consisting of epoxy resin, polyimide resin,polycarbonate resin, polyamide resin, polyester resin, polyurea resin,polyolefm resin, and polysulfone resin.

In the solder composition of the invention, the epoxy resin preferablyis selected from a group consisting of bisphenol A-type epoxy resin,bisphenol F-type epoxy resin, novolak-type epoxy resin, alicyclic epoxyresin, and their mixtures.

Also, preferably the solder composition of the invention contains analcohol. The alcohol preferably is a polyalcohol.

In the solder composition of the invention, the organic carboxylic acidpreferably is selected from a group consisting of saturated aliphaticdicarboxylic acid, unsaturated aliphatic dicarboxylic acid,cycloaliphatic dicarboxylic acid, amino group-containing carboxylicacid, hydroxyl group-containing carboxylic acid, heterocyclicdicarboxylic acid, and their mixtures. The organic carboxylic acidpreferably has a melting point of from 130 to 220° C.

The total content of the epoxy resin and the organic carboxylic acid inthe flux preferably is from 70 to 100% by mass of the flux. The contentof the alcohol therein preferably is from 0 to 30% by mass. The epoxyresin and the organic carboxylic acid preferably are so formulated inthe flux that the carboxyl group is from 0.8 to 2.0 equivalents relativeto 1.0 equivalent of the epoxy group therein.

Since the solder composition of the invention contains a lead-free SnZnalloy, its melting point is lower than that of other lead-freeSn-containing alloys such as SnAg and SnSb. As a result, it may reducethermal damage of electronic parts that are not resistant to heat inreflow soldering with it. In addition, since it contains an epoxy flux,it is free from problems of substrate corrosion and migration, and doesnot cause curing failure in resin encapsulation. It does not requireflux removal. Further, the solder composition of the invention isspecifically designed so that Zn in the SnZn does not react with theorganic carboxylic acid component in the flux to cause changes with timein the viscosity, the printability and the solderability of thecomposition. Therefore, even when stored for a long period of time afterprepared, the solder composition still has long-lasting goodsolderability, that does not undergo such changes with time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As so mentioned hereinabove, the solder composition of the invention isa solder composition containing a lead-free SnZn alloy and a solder fluxthat contains at least an epoxy resin and an organic carboxylic acid.The lead-free SnZn alloy has a lower melting point than other lead-freeSn-based alloys such as SnAg and SnSb, and, as a result, whenever usingit for reflow soldering electronic parts that are not resistant to heat,thermal damage to the parts is reduced. In addition, since the soldercomposition of the invention contains an epoxy flux, it does not causesubstrate corrosion and migration, does not cause curing failure inresin encapsulation, and does not require flux removal.

The lead-free SnZn alloy includes, but is not limited to SnZn, SnZnBiand SnZnIn. Preferably, the SnZn alloy comprises Sn as its essentialingredient, containing from 5 to 15% by mass of Zn and at most 10% bymass of Bi or In, and has a melting point of from about 180 to 210° C.More preferred are a eutectic composition of Sn9Zn (m.p. 199° C.), andSn8Zn3Bi (m.p. 197° C.), Sn8Zn6Bi (m.p. 194° C.) and Sn9Zn4In (m.p. 190°C.).

The epoxy resin for the flux component is liquid at room temperature(25° C.). It acts to mix with the solid (powder) solder component togive cream solder. It further acts to cure with the organic carboxylicacid in soldering to thereby convert the excess carboxylic acid into acured product that does not require flux removal. The cured epoxy resinproduct does not interfere with the adhesiveness of encapsulation resinto a parts-soldered printed circuit board, even when the board is notwashed but directly subjected to resin encapsulation, and, in addition,its insulation is good. Preferably, the epoxy resin is selected frombisphenol A-type epoxy resin, bisphenol F-type epoxy resin, novolak-typeepoxy resin, alicyclic epoxy resin, and their mixtures. More preferably,it is a bisphenol A-type epoxy resin.

The organic carboxylic acid for the flux component acts as an activator.That is, while used in soldering, it acts to remove the oxidized film onthe conductor pattern (e.g., copper) of a printed circuit board. It alsoacts to remove the oxidized film on the solder alloy surface so that theconductor pattern may be well wetted with the solder. In addition, theorganic carboxylic acid acts to cure with the epoxy resin. The flux foruse in the invention does not require any other activator (e.g., amineor halide activator) than the organic carboxylic acid therein. In thesolder composition of the invention, the organic carboxylic acidpreferably has a melting point of from 130 to 220° C. Of the organiccarboxylic acids mentioned below, more preferred are those having amelting point of from 130 to 220° C. The reason such organic carboxylicacids having a melting point of from 130 to 220° C. are preferred foruse in the invention, is that carboxylic acids having a melting pointlower than 130° C. may melt before the conductor surface of the printedcircuit board is activated. If the acid melts, Zn in the SnZn alloy mayreact with the carboxylic acid before the conductor surface activation.If such a reaction occurs, the wettability of the conductor surface willbe poor. On the other hand, carboxylic acids having a melting pointhigher than 220° C. could hardly melt during solder reflowing, andtherefore could not activate conductors and could not cure with epoxyresin.

The solder compositions of the above-mentioned embodiments of theinvention are described in more detail below.

The first embodiment of the solder composition of the invention containsa lead-free SnZn alloy and a solder flux that contains at least an epoxyresin and an organic carboxylic acid. The organic carboxylic acid isdispersed in the solder composition as a solid at room temperature (25°C.).

At this temperature, the organic carboxylic acid that reacts with the Znin the SnZn alloy is dispersed as a solid powder. Its contact with theZn therefore is reduced to retard its reaction with the Zn, so thatduring storage, the solder composition hardly changes over time. Whenthe solder composition is heated for soldering, the organic carboxylicacid melts before the SnZn alloy melts and, as a result, the aciddiffuses in the solder composition. This increases its contact with theSnZn alloy or with the object face to be soldered, therefore activatingthe solder powder and the object face to be soldered. At the same time,on the other hand, the epoxy resin cures with the organic carboxylicacid, and the flux containing them can therefore maintain its intrinsicfunction.

In flux preparation, in general, a carboxylic acid that serves as anactivator is heated and melted with other flux components, then stirredand cooled to give a uniform flux. According to this process, however,the carboxylic acid is liquid at room temperature like epoxy resin, andan SnZn alloy solder that comprises a flux with such a liquid carboxylicacid may undergo frequent contact between the carboxylic acid and Zn andthe solder composition of the type will therefore undergo theabove-mentioned changes over time. Contrary to this, the process forflux preparation is not employed in the invention. In the invention, anorganic carboxylic acid-containing flux is mixed with a SnZn alloy atroom temperature to prepare the solder composition of the invention.Accordingly, in the invention, the organic carboxylic acid can bedispersed in the thus-produced solder composition as a solid, and as somentioned hereinabove, the solder composition of the invention does notchange with time.

The organic carboxylic acid to be used in the above-mentioned embodimentis preferably selected from saturated aliphatic dicarboxylic acid,unsaturated aliphatic dicarboxylic acid, cycloaliphatic dicarboxylicacid, amino group-containing carboxylic acid, hydroxyl group-containingcarboxylic acid, heterocyclic dicarboxylic acid, and their mixtures.Concretely, it includes succinic acid, glutaric acid, adipic acid,azelaic acid, dodecane-diacid, itaconic acid, mesaconic acid,cyclobutane-dicarboxylic acid, cyclohexane-dicarboxylic acid,cyclohexene-dicarboxylic acid, cyclopentane-tetracarboxylic acid,methyladipic acid, L-glutamic acid, citric acid, malic acid, tartaricacid, pyrazine-dicarboxylic acid, diglycolic acid, phenylene-diaceticacid, catechol-diacetic acid, thiopropionic acid, thiodibutylic acid anddithioglycolic acid.

The second embodiment of the solder composition of the inventioncontains a lead-free SnZn alloy and a solder flux that contains at leastan epoxy resin and an organic. carboxylic acid, wherein the organiccarboxylic acid has a molecular weight of from 100 to 200 g/mol. Theorganic carboxylic acid is characterized in that it reacts with Zn inSnZn to form a salt, in the manner mentioned below.

Conventional rosin flux contains, as the active ingredient thereof, arosin having a large molecular weight (about 300 to 1000 g/mol).Therefore, with time, the Zn in SnZn may react with the rosin in theflux to thereby form a salt having a large molecular weight. As aresult, the changes with time in viscosity, printability andsolderability of cream solder that comprise the flux of this type areremarkable. However, when an epoxy flux that contains an organiccarboxylic acid having a low molecular weight is used in solder, thenthe solder may be prevented from changing with time since the organiccarboxylic acid may react with Zn to form a salt having a low molecularweight. In addition, since such a low-molecular-weight salt of thelow-molecular-weight carboxylic acid with Zn is previously formed, thesolder composition is prevented from being oxidized or being reactedwith any other component while stored. Therefore, it is stable andundergoes few changes with time.

The molecular weight of the “organic carboxylic acid having a lowmolecular weight” falls between 100 and 200 g/mol, as so mentionedhereinabove. The reason why the molecular weight of the organiccarboxylic acid is defined to fall between 100 and 200 g/mol is thatorganic carboxylic acids having a molecular weight smaller than 100g/mol are too reactive and they may greatly react with Zn in SnZnalloys. On the other hand, organic carboxylic acids having a molecularweight larger than 200 g/mol are also unfavorable, since the molecularweight of the salt formed of such an organic carboxylic acid and Zn inthe alloy may be too high and, if so, the solder may undergo changeswith time. More preferably, the organic carboxylic acid has a molecularweight of from 130 to 180 mol/g.

The organic carboxylic acid to be used in this embodiment has amolecular weight of from 100 to 200 g/mol, and is preferably selectedfrom a group consisting of saturated aliphatic dicarboxylic acid,unsaturated aliphatic dicarboxylic acid, cycloaliphatic dicarboxylicacid, amino group-containing carboxylic acid, hydroxyl group-containingcarboxylic acid, heterocyclic dicarboxylic acid, and their mixtures.More concretely, it includes succinic acid, glutaric acid, adipic acid,azelaic acid, itaconic acid, citraconic acid, mesaconic acid,cyclobutane-dicarboxylic acid, cyclohexane-dicarboxylic acid,cyclohexene-dicarboxylic acid, dimethylglutaric acid, methyladipic acid,L-glutamic acid, aspartic acid, citric acid, malic acid, tartaric acid,pyridine-dicarboxylic acid, pyrazine-dicarboxylic acid, diglycolic acid,phenylene-diacetic acid, thiopropionic acid, thiodibutylic acid anddithioglycolic acid. Preferred are cyclohexene-dicarboxylic acid,L-glutamic acid, dimethylglutaric acid and itaconic acid.

The organic carboxylic acid to be in the solder composition of thisembodiment may be dispersed in the solder composition as a solid at roomtemperature (25° C.), or may be dispersed as a liquid. When the organiccarboxylic acid is dispersed as a liquid at room temperature (25° C.),the carboxylic acid that serves as an activator is heated and meltedalong with any other flux component, stirred and cooled to prepare auniform flux, and then mixed with a solder alloy. So far as theabove-mentioned organic carboxylic acid having a low molecular weight isused in the flux, even the SnZn alloy solder that contains the liquidcarboxylic acid-containing flux may also be prevented from undergoingchanges with time. However, as described above, from the viewpoint ofreducing the contact between the organic carboxylic acid and Zn, it isdesirable that the organic carboxylic acid is dispersed in the soldercomposition as a solid at room temperature (25° C.). For example, anorganic carboxylic acid that is powder at room temperature is mixed withan epoxy resin to prepare a flux, and the flux may be mixed with an SnZnalloy whereby the organic carboxylic acid can be dispersed as a solid inthe resulting solder composition.

The third embodiment of the solder composition of the invention containsa lead-free SnZn alloy and a solder flux that contains at least an epoxyresin and an organic carboxylic acid, wherein the organic carboxylicacid is in microcapsules. The microcapsules are formed with a filmselected from a group consisting of epoxy resin, polyimide resin,polycarbonate resin, polyamide resin, polyester resin, polyurea resin,polyolefm resin, and polysulfone resin.

In the solder composition of this embodiment, the organic carboxylicacid that is highly reactive with the Zn component of the SnZn alloy atroom temperature is encapsulated with an inert film of, for example,polyimide or the like. In microcapsules, therefore, the organiccarboxylic acid is prevented from reacting with Zn, and the soldercomposition hardly undergoes any changes over time. However, when thesolder receives heat while reflowing for soldering, the film is broken,the organic carboxylic acid dissolves out and activates the object faceto be soldered and the solder powder. At the same time, the epoxy resincures with the organic carboxylic acid, and the flux containing them cantherefore perform its intrinsic function.

The microcapsule structure is not specifically defined as to how it maybe constructed (the mode of microencapsulation). For example, it may beconstructed by any known method of microencapsulation, such asinterfacial polymerization, in-liquid drying, spray-drying or vacuumvapor deposition.

The organic carboxylic acid to be used in this embodiment preferably isselected from a group consisting of saturated aliphatic dicarboxylicacid, unsaturated aliphatic dicarboxylic acid, cycloaliphaticdicarboxylic acid, amino group-containing carboxylic acid, hydroxylgroup-containing carboxylic acid, heterocyclic dicarboxylic acid, andtheir mixtures. The carboxylic acid is coated with a film of polyimideor the like to construct the microcapsules.

The fourth embodiment of the invention is a solder compositioncontaining a lead-free SnZn alloy and a solder flux that contains atleast an epoxy resin and an organic carboxylic acid, wherein the SnZnalloy is in microcapsules that are formed with a film selected from agroup consisting of epoxy resin, polyimide resin, polycarbonate resin,polyamide resin, polyester resin, polyurea resin, polyolefm resin, andpolysulfone resin.

In the solder composition of this embodiment, the SnZn alloy that, hasas its constitutive component, Zn highly reactive with an organiccarboxylic acid at room temperature, is covered with an inert film.Therefore, the Zn is protected from reacting with the carboxylic acid,and the solder composition undergoes hardly any changes with time.However, when the solder reflow receives heat while it is used forsoldering, the film is broken and the solder alloy powder dissolves outof it, and when the thus-heated solder flow reaches a temperature higherthan the melting point of the solder alloy, the solder melts to enablesoldering.

The microcapsule structure is not specifically defined so far as themode of the microencapsulation. For example, it may be constructed byany known method of microencapsulation, such as interfacialpolymerization, in-liquid drying, spray-drying or vacuum vapordeposition.

In the solder composition of the invention, the flux componentpreferably contains an alcohol as a solvent thereof. The alcohol solventdissolves the carboxylic acid in the flux to thereby lower the viscosityof the flux. In addition, the epoxy resin in the flux may react with thealcohol, and the alcohol does not remain as a residue. However, evenwhen the flux of the invention does not contain an alcohol, it stillapplies to lead-free solder. Alcohols usable in the solder flux of theinvention include methyl alcohol, ethyl alcohol, propyl alcohol, butylalcohol, isobutyl alcohol, amyl alcohol, isoamyl alcohol, allyl alcohol,cyclohexanol, as well as polyalcohols such as ethylene glycol,diethylene glycol, triethylene glycol, tetraethylene glycol, propyleneglycol, octylene glycol, polyethylene glycol, propanediol, and glycerin,and mixtures thereof. Preferred are polyalcohols; and more preferred areethylene glycol, diethylene glycol, triethylene glycol, tetraethyleneglycol, propylene glycol, polyethylene glycol, propanediol and glycerin.

If desired, the flux may contain other additives such as thixotropicacid, chelating agent, defoaming agent, surfactant and antioxidant.Regarding the amount of these additives in the flux, it is desirablethat the thixotropic agent accounts for from 0 to 5% by weight of theflux, the chelating agent for from 0 to 5% by mass, the defoaming agentfor from 0 to 1% by mass, the surfactant for from 0 to 2% by weight, andthe antioxidant for from 0 to 3% by mass.

Preferably, the solder composition is so designed that the total amountof the epoxy resin and the organic carboxylic acid in the flux is from70 to 100% by mass, and the amount of the alcohol is from 0 to 30% bymass, and the epoxy resin and the organic carboxylic acids are soformulated in the flux that the carboxyl group is from 0.8 to 2.0equivalents relative to 1.0 equivalent of the epoxy group therein. Thereason that the carboxyl group is preferred at from 0.8 to 2.0equivalents relative to 1.0 equivalent of the epoxy group in the flux,is that, if the carboxyl group is smaller than 0.8 equivalents, theactivity of the carboxylic acid will be low and the solder wettabilitythereby will be lowered. If, on the other hand, the carboxyl group islarger than 2.0 equivalents, too much solid carboxylic acid would worsenthe flowability of the flux and the solder wettability thereby will belowered. Preferably, the blend ratio of the epoxy resin and the organiccarboxylic acid is so controlled that the carboxyl group is from 1.0 to1.3 equivalents relative to 1.0 equivalent of the epoxy group, morepreferably the carboxyl group is 1.0 equivalent. The reason the totalamount of the epoxy resin and the organic carboxylic acid in the flux ispreferred at from 70 to 100% by mass of the flux is that, if it issmaller than 70% by mass, the activity of the carboxylic acid will below and the solder wettability thereby will be lowered. The reason thatthe amount of the alcohol is from 0 to 30% by mass of the flux is that,if the alcohol amount is larger than 30% by weight, it causes curingfailure in encapsulation with resin especially with silicone gel.

Preferably, the total amount of the epoxy resin and the organiccarboxylic acid is from 75 to 85% by mass of the flux, and the amount ofthe alcohol is from 15 to 25% by mass thereof. More preferably, thetotal amount of the epoxy resin and the organic carboxylic acid is 77%by mass of the flux, and the alcohol amount is 23% by mass thereof. Whenthe flux to be in the solder composition of the invention is formulatedas described above, it evades several problems. That is, most of thecarboxylic acid serving as an activator is not consumed for the curingreaction with the epoxy resin before the SnZn alloy melts, as wouldprevent the carboxylic acid from maintaining its activity. Moreover, theflux does not lose its flowability, as would cause the solderwettability to be lowered.

Soldering with the solder composition of the invention that can beattained with no flux removal from the composition is described below.When the solder composition of the invention is used for soldering, forexample, for reflow soldering of electronic parts, the epoxy flux firstbegins to react before the lead-free SnZn solder therein melts. Theactivator, organic carboxylic acid, cleans the object face to be bondedby soldering. Next, with the increase in its temperature, the lead-freesolder melts to solder electronic parts to the conductor pattern of aprinted circuit board. Even at this stage, the flux curing reactioncontinues. Then, almost at the same time that the soldering is finished,or when heated after the soldering to cure the encapsulation resin), thereaction is terminated. At this time, the cured epoxy resin covers thesoldered area to thereby reinforce the bonding part.

Some excess carboxylic acid-containing epoxy flux residue may remain inthe soldered area on the printed circuit board. However, no washing isrequired. The printed circuit board is directly encapsulated with resin(e.g., epoxy resin, silicone gel) around the bonded parts, whereupon thecarboxylic acid still remaining in the flux residue reacts with theencapsulation resin. As a result, almost all the carboxylic acid isconsumed by the curing reaction and therefore is no more active forcorrosion. In addition, the main component, epoxy resin in the epoxyflux, firmly bonds to the encapsulation resin. Accordingly, when theepoxy flux of this type is used, it ensures good solder wettability.Even though the flux residue is not removed after used in soldering, theinsulation reliability is assured with no failure in curing ofencapsulation resin.

EXAMPLES

The solder composition of the invention is described concretely withreference to the following Examples and Comparative Examples.

Example 1

4.42 g of cis-4-cyclohexene-1,2-dicarboxylic acid (m.p. 167° C.;molecular weight 170) was ground in a mortar into fine powder. Thispowder was added to a mixture of 4.33 g of triethylene glycol and 10 gof epoxy resin AER260 (bisphenol A-type epoxy resin having an epoxyequivalent of 192 g/eq, by Asahi Kasei Epoxy), at room temperature (25°C.), and mixed to prepare a flux. In the flux, the epoxy resin and thecarboxylic acid were so formulated that the epoxy group could be 1equivalent relative to 1 equivalent of the carboxyl group. The flux wasadded to an alloy of Sn8Zn3Bi at room temperature (25° C.) and mixed toprepare a cream solder composition. The solder alloy content of thesolder composition was 88% by mass. Sincecis-4-cyclohexene-1,2-dicarboxylic acid in the cream solder is alow-molecular-weight organic carboxylic acid, the carboxylic acidreacted with Zn to form a low-molecular-weight salt. The carboxylic acidwas dispersed in the solder composition as a solid at room temperature.

One hour after preparation of the solder composition, the viscosity ofthe solder composition was measured at 230 Pa·s. After storing thesolder composition in a refrigerator (5° C.) for 3 months, the viscosityof the solder composition was 240 Pa·s, thus almost the same as that ofthe fresh composition. This confirmed that the solder compositionchanged little over time. On the other hand, the solder wettability wasgood, and even though the flux residue was not actively removed aftersoldering, no curing failure occurred in encapsulation with siliconegel. Not requiring flux removal, the lead-free solder ensured goodsolderability.

Example 2

Soldering was tried in the same manner as in Example 1, for which,however, 3.83 g of L-glutamic acid (m.p. about 200° C.; molecular weight147) was used in place of cis-4-cyclohexene-1,2-dicarboxylic acid inExample 1. Since L-glutamic acid in the cream solder is alow-molecular-weight organic carboxylic acid, the carboxylic acidreacted with Zn to form a low-molecular-weight salt. The carboxylic acidwas dispersed in the solder composition as a solid at room temperature.

One hour after this solder composition was prepared, the viscosity ofthe solder composition was measured at 260 Pa·s. After the soldercomposition had been stored in a refrigerator (5° C.) for 3 months, theviscosity of the solder composition was 265 Pa·s, thus almost the sameas that of the fresh composition. This confirms that the soldercomposition had changed little over time. On the other hand, the solderwettability was good, and even though the flux residue was not activelyremoved after soldering, no curing failure occurred in encapsulationwith silicone gel. Not requiring flux removal, the lead-free solderensured good solderability.

Example 3

Soldering was tried in the same manner as in Example 1, for which,however, 4.16 g of 2,2-dimethylglutaric acid (m.p. 84° C.; molecularweight 160) that had been formed into microcapsules was used in place ofcis-4-cyclohexene-1,2-dicarboxylic acid of Example 1. Epoxy resin notreactive with Zn at room temperature (25° C.) was used for the shellmaterial of the microcapsules, and the acid was formed intomicrocapsules through interfacial polymerization. The size of themicrocapsules was 2 μm; and the capsule wall thickness thereof was 0.2μm.

One hour after the solder composition was prepared, the viscosity of thesolder composition was measured at 250 Pa·s. After the soldercomposition had been stored in a refrigerator (5° C.) for 3 months, theviscosity of the solder composition was 250 Pa·s, thus the same as thatof the fresh composition. This confirmed that the solder composition hadchanged little over time. On the other hand, the solder wettability wasgood, and even though the flux residue was not actively removed aftersoldering, no curing failure occurred in encapsulation with siliconegel. Not requiring flux removal, the lead-free solder ensured goodsolderability.

Comparative Example 1

5.80 g of triethylene glycol was added to 9.32 g of benzoyl-D-tartaricacid (m.p. 155° C.; molecular weight 358), and phthalic acid was heatedat about 130° C. and dissolved. Next, this was cooled to 100° C. orlower, and then 10 g of epoxy resin AER260 (bisphenol A-type epoxy resinhaving an epoxy equivalent of 192 g/eq, by Asahi Kasei Epoxy) was addedto it and stirred to provide a flux. In the flux, the epoxy resin andthe carboxylic acid were so formulated that the epoxy group could be 1equivalent relative to 1 equivalent of the carboxyl group. The flux wascooled to room temperature (25° C.), and added to an alloy of Sn8Zn3Biand mixed to prepare a cream solder composition. The solder alloycontent of the solder composition was 88% by mass. Sincebenzoyl-D-tartaric acid is liquid at room temperature in the soldercomposition, it readily reacted with the Zn in the solder alloy. Whenreacted with Zn, the acid formed a salt having a large molecular weight.

One hour after preparation of this solder composition, the viscosity ofthe solder composition was measured at 260 Pa·s. After being stored in arefrigerator (5° C.) for 10 days, the solder composition was no morecreamy, but formed a solid. It was thus apparent that the soldercomposition had changed over time.

1. A lead-free cream solder composition which is printable comprising: aSnZn alloy which is lead-free, a solder flux comprised of an epoxyresin, microcapsules comprising organic carboxylic acid particlesencapsulated with a resin selected from a group consisting of epoxy,polyimide, polycarbonate, polyamide, polyester, polyurea, polyolefin,and polysulfone resins, and a solvent which is a polyalcohol so thatreactivity of zinc with the organic carboxylic acid is suppressed andviscosity and solderability of the cream solder are stabilized.
 2. Thesolder composition according to claim 1, wherein the epoxy resin isselected from a group consisting of bisphenol A-type epoxy resin,bisphenol F-type epoxy resin, novolak-type epoxy resin, alicyclic epoxyresin, and their mixtures.
 3. The solder composition according to claim1, wherein the organic carboxylic acid is selected from a groupconsisting of saturated aliphatic dicarboxylic acid, unsaturatedaliphatic dicarboxylic acid, cycloaliphatic dicarboxylic acid, aminogroup-containing carboxylic acid, hydroxyl group-containing carboxylicacid, heterocyclic dicarboxylic acid, and their mixtures.
 4. The soldercomposition according to claim 1, wherein the organic carboxylic acidhas a melting point ranging from 130 to 220° C.
 5. The soldercomposition according to claim 1, wherein the total content of the epoxyresin and the organic carboxylic acid in the solder flux ranges from 70to 100% by mass of the solder flux, the content of the polyalcoholtherein ranges from 0 to 30% by mass, and the epoxy resin and theorganic carboxylic acid are so formulated in the solder flux that thecarboxylic acid ranges from 0.8 to 2.0 equivalents relative to 1.0equivalent of the epoxy resin therein.
 6. A lead-free cream soldercomposition which is printable, comprising: a solder flux comprised ofan epoxy resin, an organic carboxylic acid, and a solvent which is apolyalcohol; and microcapsules comprised of particles of a SnZn alloywhich is lead-free encapsulated with a resin selected from a groupconsisting of epoxy, polyimide, polycarbonate, polyamide, polyester,polyurea, polyolefin, and polysulfone resins so that reactivity of zincwith the organic carboxylic acid is suppressed and viscosity andsolderability of the cream solder are stabilized.
 7. The soldercomposition according to claim 6, wherein the epoxy resin is selectedfrom a group consisting of bisphenol A-type epoxy resin, bisphenolF-type epoxy resin, novolak-type epoxy resin, alicyclic epoxy resin, andtheir mixtures.
 8. The solder composition according to claim 6, whereinthe organic carboxylic acid is selected from a group consisting ofsaturated aliphatic dicarboxylic acid, unsaturated aliphaticdicarboxylic acid, cycloaliphatic dicarboxylic acid, aminogroup-containing carboxylic acid, hydroxyl group-containing carboxylicacid, heterocyclic dicarboxylic acid, and their mixtures.
 9. The soldercomposition according to claim 6, wherein the organic carboxylic acidhas a melting point ranging from 130 to 220° C.
 10. The soldercomposition according to claim 6, wherein the total content of the epoxyresin and the organic carboxylic acid in the solder flux ranges from 70to 100% by mass of the solder flux, the content of the a polyalcoholtherein ranges from 0 to 30% by mass, and the epoxy resin and theorganic carboxylic acid are so formulated in the solder flux that thecarboxylic acid ranges from 0.8 to 2.0 equivalents relative to 1.0equivalent of the epoxy resin therein.
 11. The solder compositionaccording to claim 1, wherein the carboxylic acid is selected from agroup consisting of cyclohexene-dicarboxylic acid and dimethylglutaricacid.
 12. The solder composition according to claim 6, wherein thecarboxylic acid is selected from a group consisting ofcyclohexene-dicarboxylic acid and dimethylglutaric acid.
 13. The soldercomposition according to claim 1, wherein the epoxy resin is selectedfrom a group consisting of bisphenol F-type epoxy resin, novolak-typeepoxy resin, alicyclic epoxy resin, and their mixtures.
 14. The soldercomposition according to claim 6, wherein the epoxy resin is selectedfrom a group consisting of bisphenol F-type epoxy resin, novolak-typeepoxy resin, alicyclic epoxy resin, and their mixtures.